This invention relates to a device and a method for retaining a mercury source in the discharge space of a low-pressure discharge lamp. The invention also relates to a lamp equipped with the device.
A wide variety of low-pressure discharge lamps are known in the art. These lamps contain small doses of mercury, which radiates under the influence of the discharge arc. The mercury may be introduced into the discharge space of the lamp in a number of ways. One possible method is the introduction of an amalgam, typically containing bismuth, e.g. a Biln or BiSnPb compound. The mercury vapour necessary for the operation of the lamp is released from the amalgam. The amalgam is optimally positioned near a cold spot of the lamp, for example near a tip of the discharge tube. Another method uses a so-called pellet, which contains liquid mercury. The mercury is released from the pellet after the sealing of the discharge space with the help of a heat treatment of the pellet. Both an amalgam or a pellet must be prevented from rolling freely about in the discharge space, as it may collide with the electrodes and it could scratch off the light emitting layer from the internal surface of the discharge vessel.
A known method to position the amalgam is to insert it into an exhaust tube of the discharge vessel. The amalgam is then held in a predetermined location with various methods. In the method disclosed in U.S. Pat. Nos. 5,629,584 and 5,434,482, the amalgam is held in place with indentations on the exhaust tube and glass balls before and after the amalgam. However, this structure has certain disadvantages. The tube section of the discharge vessel must be held in a vertical position, otherwise the glass balls and the amalgam will not remain in the desired location during the so-called tip-off, i. e. when the exhaust tube of the lamp is sealed and the remaining excess length of the tube is removed. In certain production lines, this is not always feasible, and there is a need for an amalgam retaining method where the amalgam is held in place irrespective of the orientation of the tube, which receives the amalgam.
A discharge lamp with an amalgam container is disclosed in U.S. Pat. No. 6,201,347. In this known discharge lamp, the container is held in place with the help of a resilient, coiled wire, which is attached to the container with the amalgam. The container and the coiled wire are pushed into a tube within the discharge space of the discharge lamp. The coiled wire acts as a clamping means, which substantially prevents the movement of the container within the tube.
Another discharge lamp with an amalgam container is disclosed in U.S. Pat. No. 6,137,236. In this known discharge lamp the container is held in place with the help of a resilient body, which surrounds the container with the amalgam. The resilient body is provided with radially extending portions, which press against a wall of a tube within the discharge space of the lamp. The extending portions of the resilient body keep the container in a predetermined location within the tube. When the container is not inserted in the resilient body, the radially extending portions of the body are somewhat retracted, and the resilient body may be inserted into the tube with ease. The extending portions spread when the container is pushed into the resilient body.
Though the retaining methods disclosed in U.S. Pat. Nos. 6,137,236 and 6,201,347 are practicable in any orientation of the discharge vessel, other problems remain. For various reasons, it is desirable to insert the mercury source into the discharge space only after an evacuation of the discharge vessel, and only shortly before the final sealing of the discharge vessel. However, the containers with the amalgam, as disclosed in U.S. Pat. Nos. 6,137,236 and 6,201,347, require relatively complicated equipment, if the containers must be fed into the tube in the evacuated state of the tube. Further, the containers need to be inserted into the tube in a predetermined position (orientation) relative to the tube. This requires further specialised positioning means in the feeding equipment, which must operate in vacuum. Such an equipment is complicated, hence expensive[007]Therefore, there is a need for a method for retaining a mercury source, which allows the insertion of the mercury source into the discharge space in vacuum, and which does not require complicated manufacturing facilities, and which may be integrated into all types of existing production lines in a simple manner.